AW: Another VCF Finished

[Kimmo Koli]

On Mon, 15 Apr 1996 gstopp at fibermux.com wrote:
>      ...
>      I was confused before about the difference between the different 
>      current-controlled integrator designs:
>      
>      * 3080 to cap to buffer, (-) input grounded, no feedback
>      * 3080 to cap to buffer, negative feedback from buffer into 3080
>      * 3080 to (-) input of op-amp integrator
>      
>      My understanding now is as follows: the first type is the same as the 
>      third type, in that they are both "open-loop" integrators. They are 
>      used in state-variable configurations because in that filter type the 
>      integrators are tied together with an overall feedback path to 
>      establish stability. In other words, the individual integrators have 
>      no feedback because the way they are connected together at a higher 
>      level provides the stabilization.
>      
>      The second type however differs in that it is a unity gain integrator. 
>      This means that it can be used by itself to provide a 6 dB/octave 
>      rolloff lowpass response, with no additional feedback. It is a 
>      "stand-alone" integrator. Cascading several of these in a row will 
>      simply increase the slope of the rolloff, without changing the 
>      stability of the overall network at all. In fact until some kind of 
>      feedback is added, the stability is at a maximum at all times. Note 
>      that this differs from the state-variable which is at maximum 
>      *instability* when there is no feedback.

This feedback type integrator works as a voltage-follower with frequency
response limited by the added pole. This means that there is only a very 
small voltage difference between the OTA-inputs and therefore the clipping
of the signal in the input of the OTA is avoided.

Actually the clipping is avoided only at frequencies considerably lower
than the pole frequency. At the pole frequency approx. 30% of the signal 
appears at the input of the OTA and at higher frequencies gradually all
of the signal appears at the input of the OTA.

You can handle large signals without any attenuating resistors with this 
design, so signal-to-noise ratios can be tens of desibels better than with 
the other designs. Also low frequency distortion is lower with this design.

>      ...      
>      Adding inverted feedback to an amplifer will always cause oscillation 
>      if the gain is high enough. The frequency of oscillation will be at 
>      the fastest rate that the amplifier can respond, and in a lowpass 
>      filter this is the cutoff frequency. This is also the reason that a 
>      4-pole highpass filter has no resonance control - because the 
>      oscillation will be at the fastest rate that the amplifier can respond 
>      at, and that will probably be in the hundreds of kilohertz and *not* 
>      at the cutoff frequency of the filter!!!
>      
Wrong! What about the resonating high-pass filter in Korg MS-20 ?
Oscillating occurs, when there is a phase shift of 180 degrees in the 
feedback loop and a gain of at least of one. If the oscillating amplitude
should be constant the loop gain should be exactly one. In real life
oscillating amplitude increases until some clipping occurs (and clipping
attenuates the loop gain down to one). The required phase shift need two 
poles or two zeroes or more, so oscillating occurs whether the filter is 
low-pas or high-pass.

The high-pass oscillating filter (like the MS-20) sound totally 
different, though. That's because because the oscillating level limiting 
means distorting the signal and therefore in the high-pass filter the 
oscillating signals has lot of overtones. In the low-pass filter these 
harmonic components are attennuated by the filter and sinusoidal signal
is heard at the filter output.
 
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  Kimmo Koli      			     Email: kimmo at clara.hut.fi 
  Helsinki University of Technology          URL:   http://clara.hut.fi/~kimmo
  Electronic Circuit Design Laboratory
  Otakaari 5 A 
  FIN-02150 Espoo     			     Tel:  +358 0 451 2273
  Finland      				     Fax:  +358 0 451 2269
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